(a) Technical Field
The present invention relates to a method for controlling motor torque in a hybrid electric vehicle, which can reduce mapping time and simplify control algorithm by providing a torque control compensation logic against a change in temperature of a motor (interior permanent magnet synchronous motor) of the hybrid electric vehicle.
(b) Background Art
Characteristics of inductance and permanent magnet magnetic flux of a motor for a hybrid electric vehicle are changed by heat generated by ambient temperature (engine room) and operation conditions.
Such changes affect control characteristics, such as maximum torque per ampere (MTPA) curve and field weakening control and may deteriorate torque control performance. Accordingly, in order to improve power performance and driveability of the hybrid electric vehicle, it is necessary to provide a measure to compensate for the change in torque according to the change in temperature of the motor.
One of the prior art methods of compensating for the torque change is to estimate a rotor secondary resistance with respect to an induction motor and regenerating a current command by a vector control operation, as shown in FIG. 4. For example, Japanese Patent No. 3339208 discloses a method for preventing a torque ripple of a motor generated by a change in battery current and a change in motor temperature. According to the method, during the change in battery current, the variation of battery current is detected to compensate for a current command and the compensated current command is sent to a vector control operator. During the change in motor temperature, a rotor temperature is estimated from an air amount of a motor cooling fan and a temperature of a motor housing to obtain a rotor secondary resistance, and the rotor secondary resistance is sent to the vector control operator to generate a new current command using the same.
The prior art method, however, has drawbacks in that the process of estimating the secondary resistance according to the rotor temperature is required to implement the method and, if an error occurs during the process, its control characteristics become significantly degraded.
Another prior art method is to form ‘n’ number of current control maps according to the motor temperature and then interpolate current commands (i*d,tn-1, i*q,tn-1, i*q,tn) at two reference temperatures close to an actual motor temperature when a torque command is applied so that new current commands (i*d and i*q) are generated by a current command generator based on the same, as shown in FIG. 5. According to this method, in which the current commands are interpolated by forming ‘n’ number of current control maps according to the motor temperature, the process of forming ‘n’ number of current control maps according to the temperature is required to implement the method. Moreover, in order to form one current control map, it is necessary to apply all of the current commands that a motor control unit (MCU) can combines in all operation ranges to measure an actual torque and then obtain MTPA curve and field weakening control curve. Thus, a great deal of time and effort is required to form n number of current control maps by repeating the above process n times, and the number of the current control maps that can be stored in the MCU is limited by memory capacity.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.